(A) Field of the Invention
The present invention relates to a semiconductor manufacturing process, particularly to a method for removing hard masks on gates in a semiconductor manufacturing process.
(B) Description of Related Art
SiGe is the alloy of silicon and germanium. This semiconductor material is commonly used in the integrated circuit manufacturing industry, where it is employed for producing heterojunction bipolar transistors or as a strain-inducing layer for CMOS transistors. This relatively new technology offers interesting opportunities in the manufacturing structure of mixed-signal circuit and analog circuit IC design.
As shown in FIG. 1, a polysilicon gate 12 for a small PMOS transistor and a polysilicon gate 14 for a large PMOS transistor are formed on a gate oxide layer 11 over a silicon substrate 10. In addition, a polysilicon gate 16 for a small NMOS transistor and a polysilicon gate 18 for a large NMOS transistor are formed on the gate oxide layer 11 over the substrate 10 also. Oxide hard masks 20 are formed on the polysilicon gates 12, 14, 16 and 18 to define the areas of the polysilicon gates 12, 14, 16 and 18. A silicon oxide layer 22, e.g., a tetraethoxysilane (TEOS) layer, of 40 angstroms and a silicon nitride layer 24 of 250 angstroms are formed afterwards. The silicon oxide layer 22 acts as a buffer to reduce stress that may generate between the silicon nitride layer 24 and the gates 12, 14, 16 and 18. As shown in FIG. 2, the semiconductor structure shown in FIG. 1 is subjected to anisotropic etching to form nitride spacers 26 beside the gate structure consisting of the polysilicon gate 12, 14, 16 or 18 and the oxide hard mask 20 thereon. Sequentially, recesses 28 are formed beside the gates 12 and 14 by lithography process and followed by dry etching as shown in FIG. 3. As shown in FIG. 4, SiGe blocks 30 are formed by epitaxy (epi) process at the recesses 28, and then the nitride spacers 26 are removed by phosphoric acid solution. The SiGe blocks 30 serve as sources and drains for the PMOS transistors. Because the polysilicon gates 12, 14, 16 and 18 are protected by the nitride spacers 26, SiGe will not form on the polysilicon gates 12, 14, 16 and 18 during the SiGe epi process. In FIGS. 5 and 6, a photoresist layer 32 is deposited and followed by etching back. According to the nature of photoresist deposition, the photoresist layer 32 tends to deposit with significant bulges over the large polysilicon gates 14 and 18. Due to the bulges over the large gates 14 and 18 and the loading effect, i.e., PR loading effect on a region of a larger area, the photoresist on the oxide hard masks 20 of the polysilicon gates 14 and 18 may not be removed completely. Consequently, photoresist residues 34 may occur on the hard masks 20 of the polysilicon gates 14 and 18. In FIG. 7, the oxide hard masks 20 are removed by dry etching and wet etching. Because the photoresist residues 34 substantially act as a mask in the process to remove the oxide hard masks 20, the oxide hard masks 20 are difficult to remove completely, and hard mask residues 36 are very likely to be formed on the polysilicon gates 14 and 18.
As a result, the residues of the hard mask film will significantly reduce the process window of contact etching, and therefore high contact resistance issues, e.g., an Rc open issue (contact resistance is infinite), may occur.